1. Field of the Invention
This invention relates to improvements in mechanical dryers for drying thick polymer-solvent layers on a substrate. More particularly, this invention is for producing and controllably applying a gradient-temperature heated air to a substrate having a thick polymer-solvent layer in order to avoid forming bubbles in the polymer layer during drying.
2. Description of the Related Art
Conventionally, it has been recognized that the latter stage of polymer solution drying is controlled by diffusion, but the application of diffusion controlled drying on dryer design has not been fully appreciated. This is largely because experiments performed imply a strong concentration dependence for the rate of diffusion. At present it appears that the dependence is even greater than that represented by existing semi-empirical diffusion models.
The typical industrial dryer for drying a polymeric coating consists of a series of zones each with a controlled temperature and airflow rate. A high drying rate enhances the process speed but may be detrimental to the quality of a final coating because of effects such as "skinning" and boiling of the solvent. The drying of polymeric films in manufacturing situations is carried out in dryers consisting of different zones. The solution of polymers and solvents is applied to a substrate by using a coater. The substrate can be a variety of materials and surfaces. An example is a web matrix used for photoreceptor belts.
The substrate with the wet film enters a series of temperature zones, each of which is at a determined temperature by applying a controlled flow of heated air. When the design in the dryer does not allow for increased temperature or airflow, air convection dryers can be augmented by supplying energy directly to the bulk of the drying film by exposing it to some sort of radiation that can be absorbed by the film. The temperatures, airflow rates and the speed of the substrate are chosen such that the residual solvent concentration at the end of the drying process is acceptable while providing the maximum yield.
Modeling the process permits optimizing the design of the dryer and to identify potential trouble spots. One problem is the boiling of the solvent in the wet film, which can result in the formation of defects in the final product, such as bubbles. Current dryer design strategies utilize high heat transfer rates and only a few relatively long temperature zones. Generally, this type of dryer design is inefficient in maximizing solvent removal rates and generally ineffective in preventing bubble formation. These small bubble formations in polymer layer, such as a small molecule transport layer of a flexible photoreceptor belt, have been a significant problem for years.
Despite early improvements in dryers, no further progress has been made in the last few years. Recently, extensive experiments in the mechanism of small molecule transport layer drying have demonstrated that conventional dryer designs such as longer zones, air bar design, and lower temperatures, cannot solve the problem. The only way to eliminate small molecule transport layer bubbles is by careful temperature profiling of a dryer.